Absence of the OKT4 epitope on blood T cells and thymus cells in a patient with thymoma, hypogammaglobulinemia, and red blood cell aplasia

Human helper/inducer T-lymphocytes that express the T4 antigen are important in the regulation of B and T cell functions. Several epitopes of the T4 molecule have now been recognized; however, the precise role of these molecules in the function of helper/inducer T cells is unclear. We studied a patient with thymoma, hypogammaglobulinemia, and red blood cell aplasia whose blood lymphocytes and thymus cells did not express the epitope recognized by OKT4 monoclonal antibody but did display the T4 epitopes recognized by OKT4A and Leu3A monoclonal antibodies. The absence of the OKT4 epitope on the patient's thymus cells suggested that the abnormality occurred during early T cell differentiation. The patient had intact delayed hypersensitivity to 4/4 antigens, and his blood lymphocytes proliferated normally to phytohemagglutinin, concanavalin A, pokeweed mitogen, tetanus toxoid, and allogeneic cells. The patient's T cells demonstrated augmented suppressor activity that was localized to the OKT8 + population rather than to the unusual T4 subset. Irradiation abrogated suppressor activity and rendered his T cells capable of providing help for polyclonal B cell differentiation. The data emphasize the limitations of OKT4 as the sole reagent for characterizing the subset of human helper/inducer cells and demonstrate that the expression of the T4 epitope recognized by OKT4 monoclonal antibody is not required for certain helper/inducer T cell functions in vitro and in vivo.